12—the Moisture Regain of Methylcellulose and Cellulose Acetate

Measurements have been made of the moisture regains, at 65 per cent and 80 or 85 per cent r.h., of homogeneously substituted methylcelluloses and cellulose acetates of various degrees of substitution. The moisture regain, expressed as molecules of water per glucose residue, was found to increase initially and then decrease linearly with increasing degree of substitution. It is considered that the linear relation shows the effect of substitution on the hygroscopicity of fully accessible cellulose and that on extrapolation to zero degree of substitution the intercept represents the proportion of water in fully accessible unsubstituted cellulose. Thus, fully accessible cellulose contains 1·53, 1·91, and 2·29 molecules of water per glucose unit at 65 per cent, 80 per cent, and 85 per cent r.h. respectively. For any specimen of cellulose the ratio of observed moisture regain to regain of fully accessible cellulose at the same r.h., gives the proportion of its hydroxyl groups that are accessible to moisture. The values for bleached cotton, mercerized cotton and viscose rayon are about 0·4, 0·5, and 0·8 respectively.